This invention is directed to pulverizing and classifying apparatus of the type in which material to be reduced in size is introduced into one end of a cylindrical housing or casing and, by action of the horizontal axis pulverizing and classifying rotors therein, is progressively reduced in size. In the classifying zone of the apparatus, particles of the desired size and finer are removed and coarse or partially ground particles are recycled for further reduction. The pulverizer-classifier of U.S. Pat. No. 3,038,672 is exemplary of such apparatus.
It is well known that a small percentage of hard abrasive contaminant, for example 1/2 to 1 percent of a metallic oxide (for example, alumina and magnesia scale) in material being ground, can greatly reduce the capacity of the pulverizing apparatus. Such contaminants also make it more difficult to meet fine grinding specifications where all of the fine particles are desired to be of as uniform a size as possible. This is especially true if the contaminant is somewhat harder than the product being ground. In some cases, the product is not contaminated by foreign substances but in its natural state has a small percentage of extra-hard particles in it. In either case, since these particles are harder to reduce in size, they will tend to continuously recycle in the tailings return. As more and more of these harder over-size particles are recycled, the capacity and efficiency of the apparatus is reduced. Any portion of these particles which escape with the fine product may tend to be coarser than the rest of the product and make it difficult to hold fineness specifications. Generally these harder particles are more abrasive and in many cases the majority of the internal wear of the apparatus is caused by this small fraction of extra hard material. The contaminants may also be particles which are darker or otherwise different in color from the desired product such that it is preferable that they be removed. The present invention is directed toward alleviation of these problems.